1. Field of the Invention
The invention relates generally to the design of shaped charges. In particular aspects, the invention relates to improved liner design for shaped charges and the use of improved shaped charges within a wellbore in order to better penetrate oil bearing sandy formations with minimal skin damage and to reduce hydrocarbon viscosity. Such a shaped charge features a composite jet that produces a large diameter hole in the formation, barely disturbing the formation properties. Such charges will greatly benefit gravel-packing completions.
2. Description of the Related Art
Shaped charges are used in wellbore perforating guns. A shaped charge typically consists of an outer housing, an explosive portion shaped as an inverted cone, and a metal liner that retains the explosive portion within the housing. When oil-bearing sands are perforated by conventional shaped charges, the full oil-producing potential of the formation is often not realized. The perforated walls tend to get cemented over by the backflow of jet material from the impacted region. During detonation of the shaped charge, a high-velocity jet is formed which is preceded by a mushroom-shaped front end and followed by a slow-moving slug of material. As the metallic jet penetrates the surrounding oilwell casing, cement sheath, and formation, portions of the casing and formation are displaced by the metallic jet and placed into plastic back flow. This results in an area around the perforation tunnel where the material that was within the tunnel has been compressed. Because the material is compressed, it is denser and less permeable than the undisturbed in-situ rock in the formation. This decrease in permeability may be sufficient to preclude hydrocarbons from entering the perforation tunnel.
In conventional shaped charges, the liner that retains the explosive charge within the housing is typically made of a single monolithic material, principally copper, but also sometimes of tungsten, brass, molybdenum, lead, nickel, tin, phosphor bronze, or some combination of these elements. Other prior liner designs have been made from sintered copper or lightly consolidated copper powder mixed with graphite and tungsten powders. These liner designs are better suited for penetration of the wellbore casing and the formation, but cause significant skin damage to the perforation tunnel and are, therefore, not optimal for use in oil-bearing formations.
The inventors of this application have recognized this. With sandy formations, the depth of the penetration is typically not of great importance to achieving good production of the well. Sandy formations have good initial permeability. Of greater significance is the cleanliness of the perforation. The high compression and ensuing plastic flow of target material damages the original permeability of the formation, thus inhibiting the free flow of hydrocarbons into the wellbore and often necessitating drastic post perforation treatment. A perforation that results in minimal skin damage will effectively permit transmission of hydrocarbons into the wellbore.
U.S. Patent Application Publication 2003/0037692 A1 by Liu discusses the use of aluminum in shaped charges. Among the several shaped charge designs discussed are those that employ aluminum either mixed with the explosive or used as a solid liner with or without the accompaniment of a copper liner for producing a deep penetrating jet. He also discusses mixing aluminum with ferrous oxide to form the liner. In Liu's design, additional energy is released through a secondary detonation when molten aluminum reacts with an oxygen carrying substance, such as water. However, Liu's application teaches mixing of inert powder aluminum with energetic explosive. This actually reduces the available energy content per unit volume of explosive, which, in turn, reduces the likelihood of aluminum undergoing the secondary detonation inside the hollow carrier gun due to the limited air space in its interior. Once the solid slug made from the aluminum liner reaches the formation, it lodges itself into the deep narrow hole made by the aluminum or copper jet that preceded it. This rapidly cooling solid slug lodged in the perforation tunnel severely restricts, if not completely stops, the flow of hydrocarbons into the well. Reaction of the aluminum slug with the borehole water will be limited to the exposed surface of the slug, at best.
The present invention addresses the problems of the prior art.